Decision feedback equalizer and semiconductor integrated circuit

ABSTRACT

A decision feedback equalizer includes a comparator configured to output a constant voltage in a reset period and to output a differential voltage corresponding to differential input signals in an evaluation period, a latch circuit configured to hold the differential voltage in the evaluation period, and an adjuster configured to adjust a logical threshold of the latch circuit closer to the output voltage in the reset period.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2016-125556, filed on Jun. 24,2016, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a decision feedbackequalizer and a semiconductor integrated circuit.

BACKGROUND

In recent years, components such as CPUs, which configure an informationprocessing system such as a server or a computer, each has achieved anincrease in performance, and in particular, bandwidths have been greatlyimproved. In order to increase the total bandwidth of the entireinformation processing system, communication circuits to perform datacommunication between components such as CPUs have to be speeded up. Ina communication circuit that performs high-speed data communication, anequalizer to compensate the deterioration of a data signal, which occursin a communication channel, is used.

As one of examples of the equalizer, there is a decision feedbackequalizer (DFE) (see, for example, Sam Palermo, “ECEN720: High-SpeedLinks Circuits and Systems Spring 2013”). As an example illustrated inFIG. 16, the decision feedback equalizer includes an adder 1601, acomparator 1602, and a feedback filter 1603. In accordance with adetermination result of the comparator 1602, the decision feedbackequalizer changes a determination threshold of the comparator 1602 by anamount corresponding to the magnitude of inter symbol interference (ISI)that may occur in the communication circuit.

In the decision feedback equalizer, every time the comparator 1602determines an input signal, the feedback filter 1603 obtains a weightedsum, based on a determination result (d_(k)) of the comparator 1602 andcoefficient W₁, . . . , W_(n-1), and W_(n) set from the outside, and theadder 1601 adds the obtained weighted sum to a reception signal y_(k) asan offset voltage of the comparator 1602. In addition, the comparator1602 performs determination while defining an output z_(k) of the adder1601 as an input signal, thereby outputting a determination result as areception data signal. In this way, the decision feedback equalizerchanges the determination threshold of the comparator by an amount ofthe inter symbol interference. Therefore, it becomes possible tocompensate the deterioration of the data signal, caused by the intersymbol interference.

Here, in general, from a viewpoint of power consumption, a comparatorthat has a reset period for performing a reset operation and anevaluation period for performing a determination operation (a comparisonoperation) is used as the comparator used for decision feedbackequalizer. In a case where the comparator used for the decision feedbackequalizer performs the reset operation, a time-interleaved configurationin which comparators are parallelized is used. However, the resetoperation performed by the comparator results in disappearance of adetermination result output by the comparator. Therefore, in order toavoid a situation that the reset operation results in the disappearanceof the determination result of the comparator, thereby causing noprevious determination result to be reflected in a comparator thatcurrently performs a determination operation, there is proposed atechnology for adding, to a subsequent stage of the comparator, a latchcircuit to hold a determination result (see, for example, R. Payne etal., “A 6.25-Gb/s Binary Transceiver in 0.13-um CMOS for Serial DataTransmission Across High Loss Legacy Backplane Channels”, IEEE 1Solid-State Circuits, vol. 40, no. 12, pp. 2646-2657, December 2005).

SUMMARY

According to an aspect of the invention, a decision feedback equalizerincludes a comparator configured to output a constant voltage in a resetperiod and to output a differential voltage corresponding todifferential input signals in an evaluation period, a latch circuitconfigured to hold the differential voltage in the evaluation period,and an adjuster configured to adjust a logical threshold of the latchcircuit closer to the output voltage in the reset period.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating an example of a configuration of adecision feedback equalizer in a first embodiment;

FIGS. 2A and 2B are diagrams illustrating an example of a configurationof a comparator in the first embodiment;

FIG. 3 is a diagram illustrating a first example of a configuration of alatch circuit in the first embodiment;

FIG. 4 is a diagram illustrating the first example of the configurationof the latch circuit in the first embodiment;

FIG. 5 is a diagram illustrating the first example of the configurationof the latch circuit in the first embodiment;

FIG. 6 is a diagram illustrating a second example of the configurationof the latch circuit in the first embodiment;

FIG. 7 is a diagram illustrating a third example of the configuration ofthe latch circuit in the first embodiment;

FIG. 8 is a diagram illustrating a fourth example of the configurationof the latch circuit in the first embodiment;

FIG. 9 is a diagram illustrating a fifth example of the configuration ofthe latch circuit in the first embodiment;

FIG. 10 is a diagram illustrating a sixth example of the configurationof the latch circuit in the first embodiment;

FIG. 11 is a diagram illustrating a seventh example of the configurationof the latch circuit in the first embodiment;

FIG. 12 is a diagram illustrating an example of a configuration of adecision feedback equalizer in a second embodiment;

FIG. 13 is a diagram illustrating an example of a configuration of alatch circuit in the second embodiment;

FIG. 14 is a diagram illustrating an example of the configuration of thedecision feedback equalizer in the present embodiment;

FIG. 15 is a diagram illustrating an example of a configuration of asemiconductor integrated circuit in an embodiment of the presenttechnology;

FIG. 16 is a diagram illustrating an example of a decision feedbackequalizer;

FIG. 17 is a diagram for explaining a delay time;

FIG. 18 is a diagram illustrating a change in a delay time, whichcorresponds to a logical threshold of the latch circuit; and

FIG. 19 is a diagram illustrating the first example of the configurationof the latch circuit in the first embodiment.

DESCRIPTION OF EMBODIMENTS

In a decision feedback equalizer, in a case of adding a latch circuit toa subsequent stage of a comparator, a delay time of a path for feedingback a determination result turns out to increase. In addition, in thedecision feedback equalizer, a determination result output by thecomparator has to be fed back within a time period of one unit interval(UI). However, in a case where a data rate in communication becomeshigh, the time period of one UI is shortened. As a result, it isdifficult for the decision feedback equalizer to deal with a higher datarate. In one aspect, an object of the present technology is to provide adecision feedback equalizer to enable high data-rate communication.Hereinafter, embodiments of the present technology will be describedbased on drawings.

First, a delay time in a case where a latch circuit is added to asubsequent stage of a comparator in a decision feedback equalizer willbe described with reference to FIG. 17. It is assumed that thecomparator has a differential input-output configuration, two outputsignals of the comparator each output the same value (a low level in thepresent example) in a reset period for performing a reset operation, andone of the two output signals of the comparator and the other thereofare put into a high level and a low level, respectively, in response toan input signal in an evaluation period for performing a determinationoperation (a comparison operation). In addition, it is assumed that thelatch circuit outputs a result obtained by inverting an input signal.

A time period in which a clock signal has a low level is the resetperiod of the comparator, and output signals 1701 and 1702 of thecomparator each have the low level. In addition, at this time, the latchcircuit holds an output of the comparator in a previous evaluationperiod and the output signal 1703 serving as one of the output signalsof the latch circuit and the output signal 1704 serving as the otherthereof have a low level and a high level, respectively. In a case wherethe clock signal changes from the low level to a high level and entersthe evaluation period of the comparator, the output signal 1701 servingas one of the output signals of the comparator and the output signal1702 serving as the other thereof are put into the high level and thelow level, respectively. At this time, in the latch circuit, uponreceiving the output signal 1702 serving as the other of the outputsignals of the comparator, the output signal 1703 serving as one of theoutput signals of the latch circuit is put into the high level, and uponreceiving the output signal 1701 serving as one of the output signals ofthe comparator, the output signal 1704 serving as the other thereof isput into the low level.

Here, in the latch circuit, a logical threshold LTH for determiningwhether an input signal has the high level or has the low level exists.In a case where an output signal of the latch circuit is put into thehigh level in the evaluation period, an output signal of the comparatorremains at the low level even if a transition from the reset period tothe evaluation period is made. In other words, since there is no delaytime before the output signal of the comparator reaches the logicalthreshold of the latch circuit, the output signal of the latch circuitis swiftly put into the high level after a short time period T11subsequent to a transition to the evaluation period. On the other hand,in a case where the output signal of the latch circuit is put into thehigh level in the evaluation period, there is a delay time before theinput signal of the latch circuit, in other words, the output signal ofthe comparator reaches the logical threshold of the latch circuit fromthe low level. As a result, compared with a case where the output signalof the latch circuit is put into the low level, the output signal of thelatch circuit is put into the high level after a long time period T12subsequent to a transition to the evaluation period.

The delay time of the entire latch circuit is determined by a timeperiod before both the two output signals of the latch circuit aresettled. Therefore, in the example illustrated in FIG. 17, the timeperiod T12 before the output signal of the latch circuit is put into thehigh level is a main factor in determining the delay time of the entirelatch circuit. Accordingly, in a case where, in the reset period, theinput signal of the latch circuit, in other words, the output signal ofthe comparator has the low level, the logical threshold of the latchcircuit is decreased, thereby shortening a time period before the outputsignal of the comparator reaches the logical threshold of the latchcircuit, and accordingly, it is possible to reduce the delay time of theentire latch circuit. In other words, by controlling the logicalthreshold of the latch circuit so as to put the logical threshold of thelatch circuit closer to the input signal of the latch circuit in thereset period, in other words, the value of the output signal of thecomparator, it is possible to reduce the delay time of the entire latchcircuit.

Therefore, in the embodiments described below, in the decision feedbackequalizer, the logical threshold of the latch circuit added to asubsequent stage of the comparator is put closer to the value of theoutput signal of the comparator in the reset period, and accordingly, adelay time of a path for feeding back a determination result isshortened. As illustrated by, for example, a solid line 1801 in FIG. 18,by decreasing the logical threshold of the latch circuit of the decisionfeedback equalizer, it is possible to shorten the delay time of the pathfor feeding back a determination result, and accordingly, it becomespossible to perform high data-rate communication.

First Embodiment

Hereinafter, a first embodiment of the present technology will bedescribed. FIG. 1 is a diagram illustrating an example of aconfiguration of a decision feedback equalizer (DFE) in the firstembodiment. FIG. 1 illustrates a decision feedback equalizer which has atwo-parallel time-interleaved configuration, in other words, a half-rateconfiguration and which includes adders 11A and 11B, comparators 12A and12B, latch circuits 13A and 13B, and logical threshold adjusters 14A and14B. Note that, in the following description, a signal described as a“signal nameX” is a signal obtained by inverting a signal having a“signal name” (the same applies to other examples).

The adder 11A adds, to differential input signals IN and INX, offsetvoltages based on output signals OUTB and OUTBX of the latch circuit 13Band outputs results thereof as output signals S1A and S1AX. In the sameway, the adder 11B adds, to the differential input signals IN and INX,offset voltages based on output signals OUTA and OUTAX of the latchcircuit 13A and outputs results thereof as output signals S1B and S1BX.

Each of the comparators 12A and 12B is a comparator that operates insynchronization with input clock signals and that has a differentialinput-output configuration. In addition, each of the comparators 12A and12B performs a reset operation and a determination operation (acomparison operation) in a reset period and an evaluation period,respectively, in response to the input clock signals. Each of thecomparators 12A and 126 is put into the reset period in a case of thelow level of a clock signal CKA and is put into the evaluation period ina case of the high level of the clock signal CKA.

In the example illustrated in FIG. 1, the comparator 12A performs thereset operation in a case of the low level of a clock signal CLK inputas the clock signal CKA and outputs output signals S2A and S2AX eachhaving a low level. In addition, the comparator 12A performs thedetermination operation (the comparison operation) related to the outputsignals S1A and S1AX of the adder 11A in a case of the high level of theclock signal CLK input as the clock signal CKA and outputs the outputsignals S2A and S2AX, one of which is put into a high level inaccordance with a determination result and the other of which is putinto the low level in accordance therewith.

In the same way, the comparator 12B performs the reset operation in acase of the low level of a clock signal CLKX input as the clock signalCKA and outputs output signals S2B and S2BX each having a low level. Inaddition, the comparator 12B performs the determination operation (thecomparison operation) related to the output signals S1B and S1BX of theadder 11B in a case of the high level of the clock signal CLKX input asthe clock signal CKA and outputs the output signals S2B and S2BX, one ofwhich is put into a high level in accordance with a determination resultand the other of which is put into the low level in accordancetherewith.

The latch circuits 13A and 13B each operate in synchronization withinput clock signals. In a case where the clock signal CKA has the highlevel, the latch circuits 13A and 13B invert and output the outputsignals of the corresponding comparators 12A and 12B, respectively, andin a case where the clock signal CKA has the low level, the latchcircuits 13A and 13B hold, invert, and output the output signals of thecomparators 12A and 12B, respectively, at a time of a transition of theclock signal CKA from the high level to the low level. In addition, inthe latch circuits 13A and 136, in accordance with control signals CTLthat are input as control signals THC and that are output by the logicalthreshold adjusters 14A and 14B, respectively, logical thresholds, eachof which is used for determining whether an input signal has a highlevel or has a low level, are controlled.

In other words, in a case where a connected comparator is in theevaluation period, each of the latch circuits 13A and 13B fetches,inverts, and outputs the output signal of the relevant comparator. Inaddition, in a case where a connected comparator is in the reset period,each of the latch circuits 13A and 13B inverts and outputs, regardlessof an output signal of the relevant comparator, an output signal of therelevant comparator, fetched in a previous evaluation period.

In the example illustrated in FIG. 1, in a case where the clock signalCLK input as the clock signal CKA has the high level, the latch circuit13A inverts and outputs the output signals S2A and S2AX of thecomparator 12A as the output signals OUTA and OUTAX. In addition, in acase where the clock signal CLK input as the clock signal CKA has thelow level, the latch circuit 13A inverts and outputs the output signalsS2A and S2AX of the comparator 12A at a time of a transition thereoffrom the high level to the low level, as the output signals OUTA andOUTAX.

In the same way, in a case where the clock signal CLKX input as theclock signal CKA has the high level, the latch circuit 13B inverts andoutputs the output signals S2B and S2BX of the comparator 12B as theoutput signals OUTB and OUTBX. In addition, in a case where the clocksignal CLKX input as the clock signal CKA has the low level, the latchcircuit 13B inverts and outputs the output signals S2B and S2BX of thecomparator 12B at a time of a transition thereof from the high level tothe low level, as the output signals OUTB and OUTBX.

The logical threshold adjusters 14A and 14B output, to the correspondinglatch circuits 13A and 13B, respectively, the control signals CTL forcontrolling the respective logical thresholds thereof. A test forobtaining a relationship between the corresponding logical threshold anda delay time may be preliminarily performed, for example, and inaccordance with the result thereof, the control signal CTL output byeach of the logical threshold adjusters 14A and 14B may be set so as toobtain a desired delay time.

In the decision feedback equalizer illustrated in FIG. 1, in a casewhere the clock signal CLK has the high level (the clock signal CLKX hasthe low level), the comparator 12A is put into the evaluation period andthe comparator 12B is put into the reset period. In addition, in a casewhere the clock signal CLKX has the high level (the clock signal CLK hasthe low level), the comparator 12B is put into the evaluation period,and the comparator 12A is put into the reset period.

Accordingly, in the decision feedback equalizer illustrated in FIG. 1,in a case where the clock signal CLK has the high level (the clocksignal CLKX has the low level), the adder 11A, the comparator 12A, andthe latch circuit 13A perform a determination operation (a comparisonoperation) related to the input signals IN and INX, and determinationresults are output as the output signals OUTA and OUTAX. In addition, ina case where the clock signal CLK has the low level (the clock signalCLKX has the high level), the adder 11B, the comparator 12B, and thelatch circuit 13B perform a determination operation (a comparisonoperation) related to the input signals IN and INX, and determinationresults are output as the output signals OUTB and OUTBX.

FIG. 2A is a diagram illustrating an example of configurations of thecomparators 12 in the first embodiment. As illustrated in, for example,FIG. 2A, each of the comparators 12 is a double-tail latch-typecomparator including N-channel transistors M1, M3, M5, M6, M9, M10, andM11, and P-channel transistors M2, M4, M7, M8, and M12.

In a reset period T_(RST) in which the clock signal CKA has the lowlevel (a clock signal CKB has a high level), the transistors M7 and M8are put into respective on-states (respective conduction states), andthe transistors M9 and M12 are put into respective off-states(respective non-conduction states). For this reason, nodes NDN and NDPare put into respective high levels, and as illustrated in FIG. 2B, bothoutput signals OUTP and OUTN are put into respective low levels.

In addition, in an evaluation period T_(CMP) in which the clock signalCKA has the high level (the clock signal CKB has the low level), thetransistors M9 and M12 are put into the respective on-states (therespective conduction states), and the transistors M7 and M8 are putinto the respective off-states (the respective non-conduction states).For this reason, in accordance with input signals INP and INN input tothe transistors M5 and M6, respectively, the potentials of the nodes NDNand NDP begin falling, one of output signals, which corresponds to anode the potential of which reaches a bottom thereof first, is put intothe low level, and a latch is set by a latch circuit configured by thetransistors M1 to M4, thereby putting the other of the output signalsinto the high level. In a case where the potential of the input signalINP is higher than the potential of the input signal INN, the outputOUTP is put into the high level and the output signal OUTN is put intothe low level, as illustrated in FIG. 2B, for example.

Hereinafter, configurations of the latch circuits 13 in the firstembodiment will be described with reference to various examples.

First Example of Configuration

FIG. 3 is a diagram illustrating a first example of the configurationsof the latch circuits 13 in the first embodiment. The latch circuit 13illustrated in FIG. 3 includes an inverter 301 to which an input signalSIN (corresponding to the output signal S2 of the correspondingcomparator 12) is input, an inverter 302 to which an input signal SINX(corresponding to the output signal S2X of the corresponding comparator12) is input, and a holding circuit 311.

The inverter 301 is connected, via a switch 304, to a current source 303connected to a power supply potential and is connected, via a switch306, to a current source 305 connected to a reference potential. Theswitches 304 and 306 are controlled by the clock signal CKA, are putinto respective on-states (respective closed states) in a case of thehigh level of the clock signal CKA, and are put into respectiveoff-states (respective open states) in a case of the low level of theclock signal CKA. In a case where the clock signal CKA has the highlevel, the inverter 301 inverts and outputs the input signal SIN as anoutput signal SOUT (corresponding to the output signal OUT of thecorresponding latch circuit 13).

The inverter 302 is connected, via a switch 308, to a current source 307connected to the power supply potential and is connected, via a switch310, to a current source 309 connected to the reference potential. Theswitches 308 and 310 are controlled by the clock signal CKA, are putinto respective on-states (respective closed states) in a case of thehigh level of the clock signal CKA, and are put into respectiveoff-states (respective open states) in a case of the low level of theclock signal CKA. In a case where the clock signal CKA has the highlevel, the second inverter 302 inverts and outputs the input signal SINXas an output signal SOUTX (corresponding to the output signal OUTX ofthe corresponding latch circuit 13).

The holding circuit 311 includes an inverter 312, an input of which isconnected to a signal line of the output signal SOUT (the output of theinverter 301), and an inverter 313, an input of which is connected to asignal line of the output signal SOUTX (the output of the inverter 302).An output of the inverter 312 is connected to the input of the inverter313, an output of the inverter 313 is connected to the input of theinverter 312, and the inverters 312 and 313 configure a latch.

The inverter 312 is connected to the power supply potential via a switch314 and is connected to the reference potential via a switch 315. Inaddition, the inverter 313 is connected to the power supply potentialvia a switch 316 and is connected to the reference potential via aswitch 317. The switches 314, 315, 316, and 317 are controlled by theclock signal CKB, are put into respective on-states (respective closedstates) in a case of the high level of the clock signal CKB, and are putinto respective off-states (respective open states) in a case of the lowlevel of the clock signal CKB. Accordingly, in a case where the clocksignal CKB has the high level, the holding circuit 311 holds and outputsthe output signals SOUT and SOUTX by using the inverters 312 and 313.

In a case where the clock signal CKA has the high level, the latchcircuit 13 illustrated in FIG. 3 inverts and outputs the input signalsSIN and SINX as the output signals SOUT and SOUTX, respectively. Inaddition, in a case where the clock signal CKB has the high level, theoutput signals SOUT and SOUTX are held and output. Here, the same clocksignal as the clock signal CKA of the connected comparator 12 is inputto the corresponding latch circuit 13, as the clock signal CKA, and thesame clock signal as the clock signal CKB of the connected comparator 12is input thereto, as the clock signal CKB. Accordingly, in a case wherethe connected comparator 12 is in the evaluation period, thecorresponding latch circuit 13 inverts and outputs, as the outputsignals SOUT and SOUTX, the input signals SIN and SINX, respectively,which serve as the respective output signals of the correspondingcomparator 12. In addition, in a case where the connected comparator 12is in the reset period, the corresponding latch circuit 13 outputs theheld output signals SOUT and SOUTX.

In addition, in accordance with the control signal THC serving as thecontrol signal CTL output by the corresponding logical thresholdadjuster 14, the amount of current applied by each of the currentsources 303, 305, 307, and 309 is controlled. By changing, in accordancewith the control signal THC from the corresponding logical thresholdadjuster 14 in this way, the amount of current applied by each of thecurrent sources 303, 305, 307, and 309, a time factor is changed, andaccordingly, it is possible to control logical thresholds in therespective inverters 301 and 302, in other words, the logical thresholdof the corresponding latch circuit 13. Therefore, the correspondinglogical threshold adjuster 14 controls the amount of current applied byeach of the current sources 303, 305, 307, and 309 so as to put thelogical threshold of the corresponding latch circuit 13 closer to thevalue of the output signal of the corresponding comparator 12 in thereset period. Accordingly, it is possible to shorten a delay time in thecorresponding latch circuit 13, and it becomes possible to shorten adelay time of a path for feeding back a determination result in thedecision feedback equalizer, thereby enabling high data-ratecommunication.

Each of the switches 304, 306, 308, 310, 314, 315, 316, and 317 in thelatch circuit 13 illustrated in FIG. 3 is configured by a transistor,for example. A complementary switch configured by an N-channeltransistor and a P-channel transistor may be used as illustrated in FIG.4, for example, or a switch configured by one of an N-channel transistorand a P-channel transistor may be used as illustrated in FIG. 5. Notethat, in each of FIG. 4 and FIG. 5, the same symbol is assigned to aconfiguration element having the same function as that of aconfiguration element illustrated in FIG. 3.

In addition, the holding circuit 311 is not limited to theabove-described configuration and may adopt such a configuration asillustrated in, for example, FIG. 19. The holding circuit 311illustrated in FIG. 19 includes inverters 1901 and 1902 and switches1903 and 1904. An input of the inverter 1901 is connected to an outputof the inverter 301, and an output thereof is connected to a signal lineof the output signal SOUTX. An input of the inverter 1902 is connectedto an output of the inverter 302, and an output thereof is connected toa signal line of the output signal SOUT.

In addition, the output of the inverter 1901 and the input of theinverter 1902 are connected to each other via the switch 1903 controlledby the clock signal CKB, and the output of the inverter 1902 and theinput of the inverter 1901 are connected to each other via the switch1904 controlled by the clock signal CKB. The switches 1903 and 1904 areput into respective on-states (respective closed states) in a case ofthe high level of the clock signal CKB and are put into respectiveoff-states (respective open states) in a case of the low level of theclock signal CKB. Accordingly, in a case where the clock signal CKB hasthe high level, in other words, the comparator 12 connected to thecorresponding latch circuit 13 is in the reset period, the holdingcircuit 311 illustrated in FIG. 19 holds and outputs the output signalsSOUT and SOUTX. Note that while being illustrated by using an example inwhich complementary switches are each configured by an N-channeltransistor and a P-channel transistor in FIG. 19, the switches 1903 and1904 are not limited to this.

Second Example of Configuration

FIG. 6 is a diagram illustrating a second example of the configurationsof the latch circuits 13 in the first embodiment. In FIG. 6, the samesymbol is assigned to a configuration element having the same functionas that of a configuration element illustrated in FIG. 3, and theredundant description thereof will be omitted.

In the latch circuit 13 illustrated in FIG. 6, the inverter 301 isconnected, via the switch 304, to a switch group 601 including switchesconnected in parallel to the power supply potential and is connected,via the switch 306, to a switch group 602 including switches connectedin parallel to the reference potential. In addition, the inverter 302 isconnected, via the switch 308, to a switch group 603 including switchesconnected in parallel to the power supply potential and is connected,via the switch 310, to a switch group 604 including switches connectedin parallel to the reference potential.

The switches included in each of the switch groups 601, 602, 603, and604 are subjected to on-off control in accordance with the controlsignal THC serving as the control signal CTL output by the correspondinglogical threshold adjuster 14. The number of switches to be put intorespective on-states in the switch groups 601, 602, 603, and 604 ischanged in accordance with the control signal THC from the correspondinglogical threshold adjuster 14, thereby changing resistance components,and accordingly, it is possible to control logical thresholds in therespective inverters 301 and 302, in other words, the logical thresholdof the corresponding latch circuit 13. Accordingly, the correspondinglogical threshold adjuster 14 changes the number of switches to be putinto respective on-states in the switch groups 601, 602, 603, and 604and controls so as to put the logical threshold of the correspondinglatch circuit 13 closer to the value of the output signal of thecorresponding comparator 12 in the reset period. Accordingly, it ispossible to shorten a delay time in the corresponding latch circuit 13,and it becomes possible to shorten a delay time of a path for feedingback a determination result in the decision feedback equalizer, therebyenabling high data-rate communication.

Third Example of Configuration

FIG. 7 is a diagram illustrating a third example of the configurationsof the latch circuits 13 in the first embodiment. In FIG. 7, the samesymbol is assigned to a configuration element having the same functionas that of a configuration element illustrated in FIG. 3, and theredundant description thereof will be omitted.

In the latch circuit 13 illustrated in FIG. 7, the inverter 301 isconnected to the power supply potential via a switch 701 and isconnected to the reference potential via a switch 702. In addition, theinverter 302 is connected to the power supply potential via a switch 705and is connected to the reference potential via a switch 706. Theswitches 701, 702, 705, and 706 are controlled by the clock signals CKAdelayed by delay circuits 703, 704, 707, and 708, respectively, are putinto on-states (closed states) in a case of the high levels of therespective delayed clock signals CKA, and are put into off-states (openstates) in a case of the low levels of the respective clock signals CKA.

A delay amount in each of the delay circuits 703, 704, 707, and 708 iscontrolled in accordance with the control signal THC serving as thecontrol signal CTL output by the corresponding logical thresholdadjuster 14. Delay amounts in the respective delay circuits 703, 704,707, and 708 are changed in accordance with the control signal THC fromthe corresponding logical threshold adjuster 14, and different timingsof being put into on-states are adopted between switches on a powersupply potential side and switches on a reference potential side.Accordingly, it is possible to control logical thresholds in therespective inverters 301 and 302, in other words, the logical thresholdof the latch circuit 13. Therefore, the corresponding logical thresholdadjuster 14 controls the delay amounts in the respective delay circuits703, 704, 707, and 708 and controls timings at which switches are putinto respective on-states, so as to put the logical threshold of thecorresponding latch circuit 13 closer to the value of the output signalof the corresponding comparator 12 in the reset period. Accordingly, itis possible to shorten a delay time in the corresponding latch circuit13, and it becomes possible to shorten a delay time of a path forfeeding back a determination result in the decision feedback equalizer,thereby enabling high data-rate communication.

Fourth Example of Configuration

FIG. 8 is a diagram illustrating a fourth example of the configurationsof the latch circuits 13 in the first embodiment. In FIG. 8, the samesymbol is assigned to a configuration element having the same functionas that of a configuration element illustrated in FIG. 3, and theredundant description thereof will be omitted.

In the latch circuit 13 illustrated in FIG. 8, the inverter 301 isconnected to the power supply potential via a switch 801 and isconnected to the reference potential via a switch 802. In addition, theinverter 302 is connected to the power supply potential via a switch 805and is connected to the reference potential via a switch 806. Theswitches 801, 802, 805, and 806 are controlled by the clock signals CKAsupplied through low-pass filters (LPFs) 803, 804, 807, and 808,respectively, are put into on-states (closed states) in a case of therespective high levels thereof, and are put into off-states (openstates) in a case of the respective low levels thereof.

A time factor in each of the low-pass filters 803, 804, 807, and 808 iscontrolled in accordance with the control signal THC serving as thecontrol signal CTL output by the corresponding logical thresholdadjuster 14. Time factors in the respective delay circuits 803, 804,807, and 808 are changed in accordance with the control signal THC fromthe corresponding logical threshold adjuster 14, and different timingsof being put into on-states are adopted between switches on a powersupply potential side and switches on a reference potential side.Accordingly, it is possible to control logical thresholds in therespective inverters 301 and 302, in other words, the logical thresholdof the latch circuit 13. Therefore, the corresponding logical thresholdadjuster 14 controls the time factors in the respective delay circuits803, 804, 807, and 808 and controls timings at which switches are putinto respective on-states, so as to put the logical threshold of thecorresponding latch circuit 13 closer to the value of the output signalof the corresponding comparator 12 in the reset period. Accordingly, itis possible to shorten a delay time in the corresponding latch circuit13, and it becomes possible to shorten a delay time of a path forfeeding back a determination result in the decision feedback equalizer,thereby enabling high data-rate communication.

Fifth Example of Configuration

FIG. 9 is a diagram illustrating a fifth example of the configurationsof the latch circuits 13 in the first embodiment. In FIG. 9, the samesymbol is assigned to a configuration element having the same functionas that of a configuration element illustrated in FIG. 3, and theredundant description thereof will be omitted.

In the latch circuit 13 illustrated in FIG. 9, the inverter 301 isconnected to the power supply potential via a switch 901 and isconnected to the reference potential via a switch 902. In addition, theinverter 302 is connected to the power supply potential via a switch 905and is connected to the reference potential via a switch 906. Theswitches 901, 902, 905, and 906 are controlled by the clock signals CKA,duty ratios of which are controlled by duty adjusters 903, 904, 907, and908, respectively, are put into on-states (closed states) in a case ofthe high levels of the respective adjusted clock signals CKA, and areput into off-states (open states) in a case of the respective low levelsthereof.

The duty adjusters 903, 904, 907, and 908 are controlled by the controlsignal THC serving as the control signal CTL output by the correspondinglogical threshold adjuster 14. The duty ratios of the clock signals CKAare changed by the respective duty adjusters 903, 904, 907, and 908 inaccordance with the control signal THC from the corresponding logicalthreshold adjuster 14, and different timings of being put into on-statesare adopted between switches on a power supply potential side andswitches on a reference potential side. Accordingly, it is possible tocontrol logical thresholds in the respective inverters 301 and 302, inother words, the logical threshold of the latch circuit 13. Therefore,the corresponding logical threshold adjuster 14 controls the dutyadjusters 903, 904, 907, and 908 and controls timings at which switchesare put into respective on-states, so as to put the logical threshold ofthe corresponding latch circuit 13 closer to the value of the outputsignal of the corresponding comparator 12 in the reset period.Accordingly, it is possible to shorten a delay time in the correspondinglatch circuit 13, and it becomes possible to shorten a delay time of apath for feeding back a determination result in the decision feedbackequalizer, thereby enabling high data-rate communication.

Sixth Example of Configuration

FIG. 10 is a diagram illustrating a sixth example of the configurationsof the latch circuits 13 in the first embodiment. In FIG. 10, the samesymbol is assigned to a configuration element having the same functionas that of a configuration element illustrated in FIG. 3, and theredundant description thereof will be omitted.

In the latch circuit 13 illustrated in FIG. 10, the inverter 301 isconnected to the power supply potential via a P-channel transistor 1001and is connected to the reference potential via an N-channel transistor1002. In addition, the inverter 302 is connected to the power supplypotential via a P-channel transistor 1007 and is connected to thereference potential via an N-channel transistor 1008.

A gate of the transistor 1001 is controlled by the clock signal CKA thevoltage level of which is changed by a DC-cut capacitor 1003 and a biascircuit 1004, and the transistor 1001 is put into an on-state (aconduction state) in a case of the high level thereof and is put into anoff-state (a non-conduction state) in a case of the low level thereof.In addition, a gate of the transistor 1002 is controlled by the clocksignal CKB the voltage level of which is changed by a DC-cut capacitor1005 and a bias circuit 1006, and the transistor 1002 is put into anon-state (a conduction state) in a case of the high level thereof and isput into an off-state (a non-conduction state) in a case of the lowlevel thereof.

In the same way, a gate of the transistor 1007 is controlled by theclock signal CKA the voltage level of which is changed by a DC-cutcapacitor 1009 and a bias circuit 1010, and the transistor 1001 is putinto an on-state (a conduction state) in a case of the high levelthereof and is put into an off-state (a non-conduction state) in a caseof the low level thereof. In addition, a gate of the transistor 1008 iscontrolled by the clock signal CKB the voltage level of which is changedby a DC-cut capacitor 1011 and a bias circuit 1012, and the transistor1008 is put into an on-state (a conduction state) in a case of the highlevel thereof and is put into an off-state (a non-conduction state) in acase of the low level thereof.

The amount of change in a voltage level in each of the bias circuits1004, 1006, 1010, and 1012 is controlled in accordance with the controlsignal THC serving as the control signal CTL output by the correspondinglogical threshold adjuster 14. Voltage levels of the clock signalssupplied to the gates of the respective transistors 1001, 1002, 1007,and 1008 are changed in accordance with the control signal THC from thecorresponding logical threshold adjuster 14. Accordingly, it is possibleto control logical thresholds in the respective inverters 301 and 302,in other words, the logical threshold of the latch circuit 13.Therefore, the corresponding logical threshold adjuster 14 controls theamounts of change in the voltage levels, caused by the respective biascircuits 1004, 1006, 1010, and 1012, and controls so as to put thelogical threshold of the corresponding latch circuit 13 closer to thevalue of the output signal of the corresponding comparator 12 in thereset period. Accordingly, it is possible to shorten a delay time in thecorresponding latch circuit 13, and it becomes possible to shorten adelay time of a path for feeding back a determination result in thedecision feedback equalizer, thereby enabling high data-ratecommunication.

Seventh Example of Configuration

FIG. 11 is a diagram illustrating a seventh example of theconfigurations of the latch circuits 13 in the first embodiment. In FIG.11, the same symbol is assigned to a configuration element having thesame function as that of a configuration element illustrated in FIG. 3,and the redundant description thereof will be omitted.

In the latch circuit 13 illustrated in FIG. 11, the inverter 301 isconnected to the power supply potential via, for example, a currentsource 1101 and is connected to the reference potential via a currentsource 1102. The output of the inverter 301 is connected to the signalline of the output signal SOUT and the holding circuit 311 via acomplementary switch 1105 configured by an N-channel transistorcontrolled by the clock signal CKA and a P-channel transistor controlledby the clock signal CKB.

In addition, the inverter 302 is connected to the power supply potentialvia, for example, a current source 1103 and is connected to thereference potential via a current source 1104. The output of theinverter 302 is connected to the signal line of the output signal SOUTXand the holding circuit 311 via a complementary switch 1106 configuredby an N-channel transistor controlled by the clock signal CKA and aP-channel transistor controlled by the clock signal CKB.

Note that the same method as that of one of the above-mentioned first tosixth examples of a configuration only has to be arbitrarily applied tocontrol of a logical threshold in the corresponding latch circuit 13. Inaddition, while an example in which complementary switches are eachconfigured by an N-channel transistor and a P-channel transistor isillustrated in FIG. 11, switches each configured by one of an N-channeltransistor controlled by the clock signal CKA and a P-channel transistorcontrolled by the clock signal CKB may be adopted in place of thecomplementary switches.

Second Embodiment

Next, a second embodiment of the present technology will be described.FIG. 12 is a diagram illustrating an example of a configuration of adecision feedback equalizer (DFE) in the second embodiment. In FIG. 12,the same symbol is assigned to a configuration element having the samefunction as that of a configuration element illustrated in FIG. 1, andthe redundant description thereof will be omitted. The decision feedbackequalizer illustrated in FIG. 12 includes the adders 11A and 11B, thecomparators 12A and 12B, latch circuits 1201A and 1201B, and logicalthreshold adjusters 1202A and 1202B.

The latch circuits 1201A and 1201B in the second embodiment each operateout of synchronization with a clock signal. Each of the latch circuits1201A and 1202B is an SR latch circuit 1301 illustrated in FIG. 13, forexample. In the latch circuit 13 illustrated in FIG. 13, the outputsignal SIN of a corresponding one of the comparators 12A and 12B isinput as a set input, and the output signal SINX of the correspondingone of the comparators 12A and 12B is input as a reset input.

The SR latch circuit 1301 is connected to the power supply potentialvia, for example, a current source 1302 and is connected to thereference potential via a current source 1303. Note that the same methodas that of one of the above-mentioned first to sixth examples of aconfiguration in the first embodiment only has to be arbitrarily appliedto control of a logical threshold of the SR latch circuit 1301 servingas the corresponding latch circuit 13. In the same way as in theabove-mentioned embodiment, logical thresholds of the respective latchcircuits 1201A and 1201B are controlled so as to be put into the valuesof the output signals of the respective comparators 12A and 12B in thereset period. Accordingly, it is possible to shorten delay times in therespective latch circuits 1201A and 1201B, and it becomes possible toshorten delay times of paths for feeding back respective determinationresults in the decision feedback equalizer, thereby enabling highdata-rate communication. In addition, by adopting a latch circuit out ofsynchronization with a clock signal as each of the latch circuits 1201Aand 1201B, a wiring line of a clock signal does not have to be provided,and it is possible to reduce a circuit size.

While the above-mentioned first and second embodiments each illustratethe half-rate decision feedback equalizer (DFE) as an example, thepresent technology is not limited to this. The present technology may beapplied to a four-parallel time-interleaved configuration, an example ofwhich is illustrated in, for example, FIG. 14, in other words, aquarter-rate decision feedback equalizer (DFE).

FIG. 14 is a diagram illustrating another example of the configurationof the decision feedback equalizer (DFE) in the present embodiment. Aquarter-rate decision feedback equalizer illustrated in FIG. 14 has afour-parallel time-interleaved configuration and includes adders 1401A,1401B, 1401C, and 1401D, comparators 1402A, 1402B, 1402C, and 1402D, andlatch circuits 1403A, 1403B, 1403C, and 1403D. Note that, in FIG. 14,illustrations of logical threshold adjusters for controlling logicalthresholds of the respective latch circuits 1403A, 1403B, 1403C, and1403D are omitted.

The adder 1401A adds, to the differential input signals IN and INX,offset voltages based on output signals OUTD and OUTDX of the latchcircuit 1403D and outputs results thereof to the comparator 1402A. Inaddition, the adder 1401B adds, to the differential input signals IN andINX, offset voltages based on output signals OUTA and OUTAX of the latchcircuit 1403A and outputs results thereof to the comparator 1402B. Inthe same way, the adder 1401C adds, to the differential input signals INand INX, offset voltages based on output signals OUTB and OUTBX of thelatch circuit 1403B and outputs results thereof to the comparator 1402C.In addition, the adder 1401D adds, to the differential input signals INand INX, offset voltages based on output signals OUTC and OUTCX of thelatch circuit 1403C and outputs results thereof to the comparator 1402D.

Each of the comparators 1402A, 1402B, 1402C, and 1402D is a comparatorthat operates in synchronization with input clock signals and that has adifferential input-output configuration, and in response to the inputclock signals, the comparators 1402A, 1402B, 1402C, and 1402D eachperform a reset operation in the reset period and each perform adetermination operation (a comparison operation) in the evaluationperiod. Each of the comparators 1402A, 1402B, 1402C, and 1402D is putinto the reset period in a case of the low level of the clock signal CKAand is put into the evaluation period in a case of the high level of theclock signal CKA. Note that a clock signal CLK2 is a signal having a90-degree phase difference with respect to a clock signal CLK1. In otherwords, clock signals CLK1, CLK2, CLK1X, and CLK2X are four-phase clocksignals having 90-degree phase differences with respect to each other.

In a case where the input clock signal CLK has the low level, thecomparator 1402A performs the reset operation and outputs output signalsS2A and S2AX each having a low level. In addition, in a case where theclock signal CLK has the high level, the comparator 1402A performs thedetermination operation (the comparison operation) related to the outputsignals of the adder 1401A and outputs the output signals S2A and S2AXcorresponding to a determination result. In a case where the input clocksignal CLK has the low level, the comparator 1402B performs the resetoperation and outputs output signals S2B and S2BX each having a lowlevel. In addition, in a case where the clock signal CLK has the highlevel, the comparator 1402B performs the determination operation (thecomparison operation) related to the output signals of the adder 1401Band outputs the output signals S2B and S2BX corresponding to adetermination result.

In the same way, in a case where the input clock signal CLK has the lowlevel, the comparator 1402C performs the reset operation and outputsoutput signals S2C and S2CX each having a low level. In addition, in acase where the clock signal CLK has the high level, the comparator 1402Cperforms the determination operation (the comparison operation) relatedto the output signals of the adder 1401C and outputs the output signalsS2C and S2CX corresponding to a determination result. In a case wherethe input clock signal CLK has the low level, the comparator 1402Dperforms the reset operation and outputs output signals S2D and S2DXeach having a low level. In addition, in a case where the clock signalCLK has the high level, the comparator 1402D performs the determinationoperation (the comparison operation) related to the output signals ofthe adder 1401D and outputs the output signals S2D and S2DXcorresponding to a determination result.

The latch circuits 1403A, 1403B, 1403C, and 1403D each operate insynchronization with an input clock signal. In a case where the clocksignals CKA each have the high level, the latch circuits 1403A, 1403B,1403C, and 1403D invert and output the output signals of thecorresponding comparators 1402A, 1402B, 1402C, and 1402D, respectively,and in a case where the clock signals CKA each have the low level, thelatch circuits 1403A, 1403B, 1403C, and 1403D hold, invert, and outputthe output signals of the comparators 1402A, 1402B, 1402C, and 1402D,respectively, at a time of transitions of the clock signals CKA from thehigh levels to the respective low levels. In addition, in each of thelatch circuits 1403A, 1403B, 1403C, and 1403D, the logical threshold fordetermining whether an input signal has the high level or has the lowlevel is controlled in accordance with the control signal CTL input asthe control signal THC.

In the quarter-rate decision feedback equalizer configured in this way,the logical thresholds in the respective latch circuits 1403 arecontrolled in the same way as in one of the above-mentioned first tosixth examples of a configuration in the first embodiment. Accordingly,it is possible to shorten delay times in the respective latch circuits1403, and it becomes possible to shorten delay times of paths forfeeding back respective determination results in the decision feedbackequalizer, thereby enabling high data-rate communication. In addition,compared with a decision feedback equalizer having a half-rateconfiguration, it becomes possible to reduce the frequency of a clocksignal.

FIG. 15 is a diagram illustrating an example of a configuration of asemiconductor integrated circuit including the decision feedbackequalizer (DFE) in one of the above-mentioned individual embodiments.The semiconductor integrated circuit in the present embodiment includesa reception circuit 1510, which converts an input serial signal RXIN toparallel signals, and an internal circuit 1520 such as a logic circuit,which performs a processing operation upon receiving the parallelsignals (data) from the reception circuit 1510.

The reception circuit 1510 includes a differential amplifier 1511, adecision feedback equalizer 1512, a demultiplexer 1513, and a clockregeneration circuit 1514. The differential amplifier 1511 receives thedifferential input serial signal RXIN transmitted via a communicationchannel or the like. The decision feedback equalizer 1512 is thedecision feedback equalizer in one of the above-mentioned individualembodiments and determines the input serial signal. The demultiplexer1513 performs serial-parallel conversion on an output of the decisionfeedback equalizer 1512, thereby outputting parallel data signals. Theparallel data signals output by the demultiplexer 1513 are output to theinternal circuit 1520. Based on a signal output by the decision feedbackequalizer 1512, the clock regeneration circuit 1514 controls a phase ofa clock signal supplied to the decision feedback equalizer 1512.

Note that the above-mentioned embodiments each illustrate just anexample of reduction to practice in implementing the present technology,and it goes without saying that these do not allow the technical scopeof the present technology to be interpreted in a limited way. In otherwords, the present technology may be implemented in various formswithout departing from the technological thought thereof or the maincharacteristics thereof.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A decision feedback equalizer comprising: acomparator configured to output a constant voltage in a reset period andto output a differential voltage corresponding to differential inputsignals in an evaluation period; a latch circuit configured to hold thedifferential voltage in the evaluation period; and an adjusterconfigured to adjust a logical threshold of the latch circuit closer tothe output voltage in the reset period.
 2. The decision feedbackequalizer according to claim 1, wherein the latch circuit includes aninverter configured to determine whether the differential voltage is alogical high level or a logical low level, and the adjuster adjusts thelogical threshold of the latch circuit, based on the amount of currentpassed through the inverter.
 3. The decision feedback equalizeraccording to claim 2, further comprising: a first current sourceconnected to a power supply potential and the inverter; and a secondcurrent source connected to a reference potential and the inverter,wherein the adjuster controls the amount of current applied by each ofthe first current source and the second current source.
 4. The decisionfeedback equalizer according to claim 2, further comprising: a firstswitch group including a plurality of switches connected in parallel toa power supply potential and the inverter; and a second switch groupincluding a plurality of switches connected in parallel to a referencepotential and the inverter, wherein the adjuster controls the number ofswitches to be put into respective on-states in the first switch groupand the second switch group.
 5. The decision feedback equalizeraccording to claim 3, further comprising: a first switch arrangedbetween the power supply potential and the inverter; and a second switcharranged between the reference potential and the inverter, wherein thefirst switch and the second switch are put into respective off-states inthe reset period and are put into respective on-states in the evaluationperiod.
 6. The decision feedback equalizer according to claim 1, furthercomprising: the latch circuit includes an inverter configured todetermine whether the differential voltage is a logical high level or alogical low level, a first switch arranged between the power supplypotential and the inverter; and a second switch arranged between thereference potential and the inverter, wherein the adjuster adjusts thelogical threshold of the latch circuit, based on timings at which thefirst switch and the second switch are put into respective on-states. 7.The decision feedback equalizer according to claim 6, furthercomprising: a first delay circuit configured to delay a control signalfor putting the first switch into the on-state; and a second delaycircuit configured to delay a control signal for putting the secondswitch into the on-state, wherein the adjuster controls delay amounts ofthe first delay circuit and the second delay circuit.
 8. The decisionfeedback equalizer according to claim 7, wherein each of the first delaycircuit and the second delay circuit is a low-pass filter, a time factorof which is controlled by the adjuster.
 9. The decision feedbackequalizer according to claim 6, further comprising: a first dutyadjuster configured to change a duty ratio of a signal for controllingthe first switch; and a second duty adjuster configured to change a dutyratio of a signal for controlling the second switch, wherein theadjuster adjusts the duty ratios of the respective control signals forcontrolling the first switch and the second switch and controls thelogical threshold of the latch circuit.
 10. The decision feedbackequalizer according to claim 6, further comprising: a first bias circuitconfigured to change a voltage level of a signal for controlling thefirst switch; and a second bias circuit configured to change a voltagelevel of a signal for controlling the second switch, wherein theadjuster adjusts the logical threshold of the latch circuit, based onthe voltage levels of the respective signals for controlling the firstswitch and the second switch.
 11. The decision feedback equalizeraccording to claim 1, wherein the latch circuit is an SR latch circuitconfigured to output a high level or a low level in accordance with anoutput of the comparator in the evaluation period.
 12. The decisionfeedback equalizer according to claim 1, wherein the decision feedbackequalizer is a half-rate decision feedback equalizer in which the twocomparators and the two latch circuits are arranged in parallel.
 13. Thedecision feedback equalizer according to claim 1, wherein the decisionfeedback equalizer is a quarter-rate decision feedback equalizer inwhich the four comparators and the four latch circuits are arranged inparallel.
 14. A semiconductor integrated circuit comprising: a decisionfeedback equalizer configured to decide data being sampled from an inputserial signals on the basis of a clock signal; a multiplexer configuredto output a parallel signal in accordance with an output signal of thedecision feedback equalizer; and a clock recovery circuit configured tocontrol a phase of the clock signal on the basis of a received signal,wherein the decision feedback equalizer includes: a comparatorconfigured to a constant voltage in a reset period and to output adifferential voltage corresponding to differential input signals in anevaluation period, a latch circuit configured to hold the differentialvoltage in the evaluation period, and an adjuster configured to adjust alogical threshold of the latch circuit closer to the output voltage inthe reset period.